1. Field of the Invention
The present invention relates to a heat exchanger used for automobile air-conditioners, and for oil coolers for industrial use, and more particularly to a heat exchanger for such use having a core which includes tubular elements for allowing a coolant to pass, and fin members joined thereto. Hereinafter the term "aluminum" includes aluminum-base alloys unless specified to the contrary.
2. Description of the Prior Art
There are many types of heat exchangers of aluminum used for the above-mentioned applications, which have a plurality of aluminum tubes, and fins provided in spaces between one tube and the next. To facilitate the joint between the tubes and fins, recently a brazing sheet method has been introduced. Under this method the fins are previously covered with a brazing sheet, thereby enabling them to be thermally joined to the tubes in a furnace at one time.
The heat exchangers used for the abovementioned applications must be resistant to corrosion, otherwise leakage would occur through a corroded part.
To achieve the ability of corrosion resistance, the tubes or fins or both are made of different types of aluminum selected so that even if corrosion occurs on the fins it precedes the corrosion on the part of tubes, thereby protecting the tubes against earlier corrosion. Another practice is to use a non-corrosive flux, such as fluorides, which leaves no corrosive residue after the fins are brazed to the tubes.
The most common practice is to cover the surfaces of the tubes with zinc before hand, and diffuse the zinc over the surfaces thereof by heat. To cover the surfaces of the tubes with zinc, the common practice is to immerse the tubes in a zincate solution, which contains 50 to 150g/l and 200 to 400g/l of NaOH. Hereinafter this practice will be referred to as the zincate process.
However the corrosion resistance achieved by the zincate process is unstable. In general, sufficient corrosion resistance results from a relatively high concentration of zinc in the top layer such as 1 to 5% by weight. The high concentration is achieved by applying a thickness of 0.3 .mu.m or more, most safety a level of thickness of 1 .mu.m. However the zincate process cannot produce zinc layers having such thicknesses; if the working temperature is raised, the thickness of zinc layers will increase but is likely to become uneven. A thin part is liable to breakage and early corrosion.